The present invention relates generally to apparatuses and methods for measuring the viscous nature or viscosities of liquids and more particularly to a liquid viscometric apparatus and method in which viscosities of liquids of unknown viscosities (hereinafter referred to as "test liquids") are measured by causing these test liquids to flow through thin or narrow tubes.
Heretofore, in the viscometry of liquids, the values of the liquid viscosities have been determined from the liquid flow velocity and from the liquid resistance to flow. The principal methods of determining liquid viscosity values from liquid flow velocity are (1) the narrow tube method and (2) the steel ball fall method. Those of determining viscosity values from resistance are (3) the planar laminar flow method, (4) the coaxial cylinder rotation method, and (5) the cone-flat plate rotation method.
These methods have not been fully satisfactory for the following reasons. In the case of method (1), the measuring procedure requires much time since falling velocity is measured. In the case of method (2), measurement of a sample of small quantity is difficult. In the cases of methods (3) and (4), the properties of the liquid being measured undergo change because force from the outside is applied to the liquid. Furthermore, by the narrow tube method (1), because viscosity is determined from the resistance of the liquid, measurement of liquids of low viscosities is troublesome.
As means for overcoming the above described difficulties encountered in the prior art, a liquid viscometric apparatus has been previously proposed as disclosed in Japanese Utility Model Laid-Open publication No. 03-127248 (1991). In this viscometric apparatus, a narrow or thin tube such as a capillary tube is used in a circuit through which the liquid being measured is caused to flow. In the operation of this apparatus, the resistance to flow of the test liquid thus flowing through the narrow tube is not utilized. Instead, the time for the test liquid flowing through the capillary tube to move from a first point at an upstream position to a second point at a downstream position is measured, and the viscosity is calculated from this measured time. Moreover by changing the inner diameter of the capillary tube, measurement of low to high viscosities is made possible. Furthermore, in order to shorten the measurement time, differential pressure of positive pressure or negative pressure is applied to the flowing liquid.
In this previously proposed liquid viscometric apparatus described above, the time for the test liquid to travel from the above mentioned upstream first point to the downstream second point varies depending on a set suction pressure and other factors. For this reason, the measurement and computation of the viscosity in each case are carried out on the premise that differential pressure between the upstream and downstream sides of the liquid being measured is constant throughout the time period for the liquid to move from the above mentioned first point to the second point.
However it is not easy to fix this differential pressure at a set suction pressure or the like. Furthermore, even if this differential pressure could be maintained at a set suction pressure or the like, it cannot be readily reproduced accurately and positively.
Because of the nature of the method and apparatus for measuring the viscosities of liquids as described above, even a minute variation of the differential pressure of the liquid between the upstream and downstream sides causes a variation in the measured time for movement of the liquid from the first to the second points. Thus there has been the problem of error in the measured time value.